Methods suggested for fabricating thin film wiring on a substrate typically involve a number of expensive and time consuming steps. A particular example of such for circuit definition involves reactive ion etching, e.g. oxygen or combination of oxygen and fluorocarbon such as carbon tetrafluoride, through relatively thick lift-off stencil such as soluble polyamide and dielectric (e.g.--PMDA--ODA polyimide) layers.
After the reactive ion etching, backfilling with the desired metals is carried out.
This backfilling is currently done by building up individually the single circuit pattern (lines and vias) either in lift-off structures or by plating.
However, prior to the reactive ion etching, it is necessary to render the circuit pattern resistant to the reactive ion etching. Such methods include chemical silylation and preferably the use of inherent etch resistant materials, e.g. polymers containing inorganic components or inorganic materials.
After the backfilling with the metal, the lift-off mask (e.g.--soluble polyimide) is removed by dissolution in a suitable solvent. Removal of the mask in turn removes, or lifts off with it, the metal film above the mask on the undesired areas to leave the metal circuit pattern on the substrate.
The above sequence of steps; however, suffers from some deficiencies. For instance, the etch rate for reactive ion etching is not especially uniform over a relatively large substrate required for a multichip module.
The nonuniformity of the etch rate can result in variations in the dielectric thickness, which in turn, can cause deterioration in the electrical performance of a device. To overcome nonuniformity in etch rate, etch stop layers can be employed beneath the dielectric layer. The etchstop layer allows over-etching which will lead to uniform depth in the etched channels. However, the etchstop materials suggested are somewhat wanting in one or more of the properties desired. For instance, the etchstop should be resistant to the etching process, exhibit good adhesion to the dielectric and material beneath the etchstop, have high thermal stability and a thermal coefficient of expansion that closely matches that of the other layers of the structure.
In addition, lift-off processes employed can require relatively long soaking times (e.g.--several hours) in the lift-off solvent. This long soaking can result in diffusion of the solvent into the dielectric which in turn causes increase in stress that can lead to delamination of the metal layers from the dielectric.